Particulate matters (PM), i.e., one sort of exhaust substances of an internal combustion engine is reduced to be minute in amount by improvement in performance of the engine, and it becomes difficult in situation to measure the same by a conventional filter-type weighing method. Therefore, as an alternative of the filter-type weighing method, there has been developed a technique of measuring the number of PM contained in exhaust gas. As a specific system configuration thereof, there has been known a technique such that, for example, a dilution unit for diluting exhaust gas of an internal combustion engine with air etc. is provided in a front stage of a particle number measuring device so that a part of the diluted exhaust gas is led to the particle number measuring device so as to count the number of the particles contained therein (see Patent Literature 1).
The dilution unit has a basic structure such that a diluent gas flow passage is connected to an intermediate portion of a main flow passage communicating between an input terminal and an output terminal so that diluent gas is mixed with the input gas introduced through the input terminal so that the resultant mixed gas is outputted through the output terminal.
By the way, in the case where a dilution ratio is desired to be increased, although the dilution units as mentioned above are connected in series, if the dilution units are simply connected, the flow rate of the input gas to a subsequent dilution unit becomes too large to dilute the inflow gas. Therefore, as disclosed in Patent Literature 2, some or whole of the dilution units are provided with derivation flow passages branched from the respective main flow passages for deriving a part of the gas flowing inside thereof so that the flow rate of the output gas is reduced.
Therefore, each of the derivation flow passages is provided with, e.g., a critical orifice type constant flow rate instrument so that the flow rate of the derivation flow passage is made constant so as to be measured. It is noted that, the critical orifice type constant flow rate instrument is intended to have a construction such that an upstream side pressure thereof is made higher than the downstream side pressure over a prescribed ratio so that a flow velocity at a throttle portion becomes the sonic velocity. The flow rate Q thereof becomes depending only on the upstream side pressure P1 and temperature T but independent of the downstream side pressure as shown by following Formula (A):Q=600·C·(P1+0.1)·(293/T)1/2  (A)
Therefore, if the upstream side pressure and temperature are kept constant without being affected by a pulsation etc. of a suction pump, a constant flow rate can be obtained. Meanwhile, upon measurement of the upstream side pressure and temperature, the flow rate at that time can be calculated. Also, in the critical orifice type constant flow rate instrument like this, it is necessary to measure the flow rate characteristics, for example, determine C (sonic conductance) prior to use. In specific, a known flow rate is rendered to flow and the pressure and temperature at that time are measured so as to calculate C based on the measured values (this process also referred to as “calibration” hereinafter).
Thus, in the case of a gas analyzing system of this type, when the critical orifice type constant flow rate instrument is calibrated, diluent gas of a prescribed flow rate controlled by flow rate control means is led from a diluent gas flow passage of the dilution unit to the critical orifice type constant flow rate instrument so as to measure the upstream side pressure and temperature at this time. The prescribed flow rate is a flow rate that is set to the critical orifice type constant flow rate instrument in actual use.